The present invention relates to nanostrutures and particularly to solution processing methods for doping a carbon nanotube (CN).
In the field of molecular nanoelectronics, few materials show as much promise as nanotubes, and in particular carbon nanotubes, which comprise hollow cylinders of graphite. Nanotubes are made into tiny electronic devices such as diodes and transistors, depending on the electrical characteristics of the nanotube. Nanotubes are unique for their size, shape, and physical properties. Structurally a carbon nanotube resembles a hexagonal lattice of carbon rolled into a cylinder.
Besides exhibiting intriguing quantum behaviors at low temperature, carbon nanotubes exhibit at least two important characteristics: a nanotube can be either metallic or semiconductor depending on its chirality (i.e., conformational geometry). Metallic nanotubes can carry extremely large current densities with constant resistivity. Semiconducting nanotubes can be electrically switched on and off as field-effect transistors (FETs). The two types may be covalently joined by sharing electrons. These characteristics make nanotubes excellent materials for making nanometer-sized semiconductor circuits.
Current methods for preparing nanotubes rely on the random formation of both metallic and semiconducting nanotubes. Under current methods, carbon nanotube FETs are fabricated from as-grown carbon nanotubes in ambient conditions. These nanotubes show p channel conduction due to oxygen interaction at the metal-carbon nanotube interface (V. Derycke et al. Appl. Phys. Lett. 80, 2773 (2002)). The oxygen content at the metal-carbon nanotube interface can be easily changed by standard fabrication processes (e.g., any post processing involving vacuum pumping such as thin film deposition). In fact, a p-carbon-nanotube FET can be easily converted to an ambipolar or n-carbon nanotube FET via vacuum pumping.
Current methods for p-doping of carbon nanotube using gaseous NO2 require the device to be kept under a controlled environment to prevent dopant desorption. Current methods for n channel conduction of carbon nanotube FET require annealing/out-gassing oxygen at the contacts or by doping with electron-donating alkali metals (V. Derycke et al. Appl. Phys. Lett. 80, 2773 (2002)) or gases (NH3). Both require a controlled environment and the devices degrade quickly and stop functioning upon exposure to air (J. Kong et al., Science, 287, 622 (2000)).
Carbon nanotube FETs are known to be Schottky barrier (SB) FETs, whose switching is dominated by the SBs formed at the metal/nanotube interface (J. Appenzeller et al., Phys. Rev. Lett. 89,126801 (2002)) and operate as p-type FETs in air (V. Derycke et al., Appl. Phys. Lett. 80, 2773 (2002)). As gate dielectric thickness scaled down, due to the quasi one dimensional-channel of the nanotube and the ultrathin carbon nanotube body thickness, the SB can be thinned sufficiently to allow thermally-assisted tunneling of electrons or holes, and carbon nanotube FETs operate as ambipolar FETs in air. The simultaneous injection of electrons and holes into carbon nanotube channel and the exponentially deteriorating OFF current (defined as the leakage current through transistor when the conduction is switched off) with an increasing drain field (M. Radosavljevic et al., Appl. Phys. Lett. 83, 2435 (2003)) is unacceptable in a scaled FET (where the OFF current can be as high as the ON current and the transistor cannot be switched off) for potential logic gates applications. In addition, the lack of control of current carbon nanotube FET fabrication methods has resulted in carbon nanotube FETs which exhibit a large variation in the device drive current, and a device threshold voltage being too high for ultimate device scaling.
There are no known methods for reliably preparing a carbon nanotube having particular characteristics. Nor are there known methods of nanotube separation such as selective synthesis (a procedure for the selective synthesis of a metallic or a semiconducting nanotube), or post-synthesis (a procedure for the isolation of metallic tubes from semiconducting tubes or to convert metallic tubes to semiconducting tubes). Particularly, there are no known methods for p-doping of carbon nanotube FETs. Therefore, there exists a need for a system and method which provide stable and consistent doping methods for the manufacture of carbon nanotube FETs wherein such nanotubes exhibit an improved drive current, a reduced/tunable threshold voltage and a suppression of minority carrier injection in off state (i.e., transformation from an ambipolar to a unipolar transistor) and are stable in ambient conditions.